Petroleum products high in vanadium and sulfur have been used as boiler fuels, however, their combustion results in the deposition of vanadium products on the walls and/or heat exchange tubes within the boilers. The deposited vanadium compounds result in high temperature slag corrosion which is discussed in detail by Brinser et al., The Society of Naval Architect's and Marine Engineers Transactions, Volume 85, 1977 pp. 271-299. The high temperature corrosion of boilers is preceded by the formation of vanadium based slags on the interior of the boiler. A method of minimizing the formation of the vanadium based slags is to operate at combustion conditions that maximize the production of trivalent and tetravalent oxides of vanadium which have melting points of about 3,600.degree. F. which is considerably higher than the combustion temperatures obtained in these boilers. Vanadium oxides that are in the higher oxidation state, e.g. pentavalent and septavalent oxides of vanadium and complexes thereof have melting points from about 1,200.degree. to about 1,300.degree. F. Formation of these compounds during combustion is to be avoided since they melt at or below the temperature range of the combustion process and will plate out or adhere to various critical boiler parts such as heat exchange tubing and the like. It is believed that the pentavalent and septavalent vanadium oxides adhere to the boiler surfaces and the sulfur by-products of the combustion process becomes entrained in the vanadium oxides along with other by-products of combustion thereby causing the formation of slag on the tube. As the slag increases in thickness, the temperature of the coating increases, destroys the protective oxide film on the boiler tubing, after which tube components are leached into the slag. The method employed by the prior art to minimize corrosion was to reduce the amount of air in the combustion process and thereby minimize the formation of the pentavalent and septavalent low melting oxides of vanadium while maximizing the high melting trivalent and tetravalent oxides of vanadium. In any event, the prior art method of reducing the air/fuel ratio did not substantially eliminate vanadium corrosion.
The present invention obviates this and other difficulties of the prior art by removing the vanadium as a coating on a particulate substrate such as silica. The coating is preferably formed during combustion on a substrate suspended as a fluidized bed and in the presence of an oxidizing gas used to support the bed. The process has the dual advantage of producing a particle that does not agglomerate and further results in the substantially complete capture of the vanadium thereby preventing it from plating out on any of the boiler parts. This phenomenon occurs even when high air fuel ratios are used which, according to the prior art tended to produce low melting oxides of vanadium and complexes thereof that would lead one to predict that agglomeration and plating would occur followed by vanadium corrosion of critical boiler parts.
The prior art U.S. Pat. Nos. to Carpenter, 781,808; Becket, 856,325 and 856,328 and 876,313; Rathmann et al., 3,420,659 and Macrae et al., 4,099,958 describe various methods for the production of vanadium and its alloys. Rathmann et al. disclose a process of treating vanadium-bearing slags with silica, flux and a carbonaceous reducing agent by smelting to produce a primary vanadium silicide having 25-60% silicon. The primary silicide is refined with lime and a vanadium-bearing slag or a vanadium oxide to produce a vanadium alloy having a silicon content less than 20%. The prior art also discloses fluidized bed processes such as the U.S. Pat. to Malgarini et al., 4,082,545.
None of these prior art references disclose a process for producing a non-agglomerating vanadium coated particle by the combustion of a carbonaceous material containing relatively high amounts of vanadium as an impurity in an oxidizing gas.
Accordingly it is an object of the present invention to overcome these and other difficulties encountered in the prior art.
It is a further object of the present invention to provide a novel process for producing a non-agglomerating vanadium coated particle. It is a further object of the present invention to provide a process for eliminating or substantially eliminating the coating of a fire box or boiler with vanadium when burning a carbonaceous material or fuel high in vanadium and high in vanadium and sulfur such as petroleum coke, bunker oil and other petroleum products.
It is also an object of the present invention to provide a process for burning carbonaceous materials such as petroleum coke, bunker oil and other petroleum products high in vanadium or high in vanadium and sulfur with relatively large excesses of an oxygen containing gas with the substantial to complete elimination of vanadium build-ups or coatings within the firebox or boiler in which such combustion process is conducted.
These and other objects have been achieved according to the method of the present invention which will be understood more completely by reference to the appended claims, drawings and the following disclosure.